A backflow-preventing valve is required in many jurisdictions when a plumbing fixture is connected to a normally pressurized line, in particular when used with a sprayer or hand shower that could be immersed in a body of dirty water when the supply pressure fails The backflow preventer has a high-pressure port connected to the normally pressurized service or supply line, a low-pressure port connected to the fixture, and a vent port. The high- and low-pressure ports are connected together and the vent port is blocked so long as pressure at the high-pressure port is higher than that at the low-pressure port. When, for instance due to a water-main failure or a rupture in the supply line at a lower elevation, pressure in the service line drops below that in the fixture or below atmospheric, the backflow preventer connects the high-pressure port to the vent port so that air, not the water in the fixture is drawn in. The device also frequently vents the low-pressure side of the device.
Such a device is built right into the faucet seen in commonly owned U.S. Pat. No. 4,827,538 of B Heimann et al. Here the vent valve is provided immediately downstream of a mixing valve whose upstream side is connected to incoming hot- and cold-water supply lines Any modest superatmospheric pressure at the vent valve will maintain it closed, but when pressure drops too low it opens the downstream side of the mixing valve to the atmosphere.
Such an arrangement has two main disadvantages. First of all it adds somewhat to the bulk of the fixture. Designing around this extra element is difficult since it must take up some room at a critical area, particularly in a deck-mount fixture. Second it is not required in all jurisdictions, so that valves sold in these regions either are equipped with unnecessary structure that needlessly raises the valve cost, a plug is substituted for the valve, or a second version of the valve without the vent device must be produced.
It is also known from German patent document 3,708,169 to provide a backflow preventer as a separate device having high-pressure, low-pressure, and vent ports and provided with an internal accordion-pleated tube that when internally pressurized directly connects the high-pressure port with the low-pressure port. When depressurized it retracts and opens both ports to the vent port. Such an arrangement shares with the system described above the disadvantage that in the backflow-preventing position all of the ports are interconnected. While this in theory will prevent the inlet port from sucking liquid in from the outlet port, it is technically possible for the entire housing of the valve to become flooded so that indeed some liquid does flow back into the inlet port. Thus a main disadvantage of this system is that if it is mounted below the liquid level, for instance, of a sink it will not work properly, as backflow can completely flood it.